BOILING HEAT TRANSFER

BOILING HEAT TRANSFER Heat Transfer BOILING HEAT TRANSFER The formation of steam bubbles along a heat transfer surface has a significant effect on the overall heat transfer rate. EO 1.15 DESCRIBE the process that occurs in the following regions of the boiling heat transfer curve: a. Nucleate boiling b. Partial film boiling c. Film boiling d. Departure from nucleate boiling (DNB) e. Critical heat flux Boiling In a nuclear facility, convective heat transfer is used to remove heat from a heat transfer surface. The liquid used for cooling is usually in a compressed state, (that is, a subcooled fluid) at pressures higher than the normal saturation pressure for the given temperature. Under certain conditions, some type of boiling (usually nucleate boiling) can take place. It is advisable, therefore, to study the process of boiling as it applies to the nuclear field when discussing convection heat transfer. More than one type of boiling can take place within a nuclear facility, especially if there is a rapid loss of coolant pressure. A discussion of the boiling processes, specifically local and bulk boiling, will help the student understand these processes and provide a clearer picture of why bulk boiling (specifically film boiling) is to be avoided in nuclear facility operations. Nucleate Boiling The most common type of local boiling encountered in nuclear facilities is nucleate boiling. In nucleate boiling, steam bubbles form at the heat transfer surface and then break away and are carried into the main stream of the fluid. Such movement enhances heat transfer because the heat generated at the surface is carried directly into the fluid stream. Once in the main fluid stream, the bubbles collapse because the bulk temperature of the fluid is not as high as the heat transfer surface temperature where the bubbles were created. This heat transfer process is sometimes desirable because the energy created at the heat transfer surface is quickly and efficiently "carried" away. HT-02 Page 40 Rev. 0